This application claims priority from Japanese Patent Application Reference No. 11-176681, filed Jun. 23, 1999.
The present invention relates generally to a charged particle beam apparatus and specifically to techniques for automatically adjusting for astigmatism.
An electron microscope is used as an automatic inspection system for inspecting and measuring a fine circuit pattern created on a substrate such as a semiconductor wafer. In the case of a defect inspection, an electron beam image detected by a scanning electron microscope is acquired and compared with a reference image used as a comparison standard. In a measurement of a hole diameter or a fine circuit pattern used in monitoring and setting a manufacturing process condition of a semiconductor device, measurement of a length is based on image processing of an electron beam image detected from the scanning electron beam microscope. In an inspection to detect a defect of a pattern by comparison of an electron beam image of the pattern with a reference image and in a measurement of a line width of a pattern by processing an electron beam image, the quality of the electron beam image greatly affects the reliability of the result of the inspection and the measurement.
The quality of an electron beam image deteriorates due to causes such as astigmatism of the electron beam system and degradation of the resolution attributed to defocusing. A poor quality electron beam image causes the inspection sensitivity and the performance of the length measurement to deteriorate. In addition, in such an image, the pattern width varies and a result of detection of an image edge can not be obtained in a relatively stable manner. In consequence, results of measuring the wire width of a pattern and the diameter of a hole with such a poor quality beam will often be unsatisfactory.
Conventionally, the focal point and the astigmatism of an electron beam optical system are adjusted by properly changing a control current of an objective lens and control currents of 2 coil sets each comprising a plurality of astigmatism correction coils while visually observing an electron beam image. To be more specific, the focal point is adjusted by varying the convergence height of a beam. The convergence height of a beam is changed by adjusting a current flowing through the objective lens.
While there are perceived advantages, it can take time to execute the conventional technique of adjusting a control current of an objective lens and control currents of 2 coil sets each comprising a plurality of astigmatism correction coils while visually observing an electron beam image as described above. In addition, the conventional techniques often require that the surface of a sample be scanned by using an electron beam several times. As a result, it is quite within the bounds of possibility that a problem of a damage inflicted on the sample arises. In addition, since in conventional systems, adjustments are often carried out manually, the result of the adjustment varies from operator to operator. Moreover, the astigmatism and the focal position can change with time. It is thus necessary to adjust the astigmatism and the focal position periodically by manual operations in order to carry out an automatic inspection and an automatic measurement of a length.
What is needed are automated techniques for controlling electron beams.
According to the invention, techniques for automatically adjusting for astigmatism in a charged particle beam apparatus are provided. Embodiments according to the present invention can provide a charged particle beam apparatus and an automatic astigmatism adjustment methods capable of automatically correcting astigmatism and a focal point in a relatively short period of time by finding a plurality of astigmatism correction quantities and a focal point correction quantity in a single operation from a relatively small number of 2 dimensional images. Specific embodiments can perform such automatic focusing while minimizing damages inflicted on subject samples. Embodiments include, among others, a charged particle optical system for carrying out an inspection, a measurement and a fabrication with a relatively high degree of accuracy by using a charged particle beam.
Numerous benefits are achieved by way of the present invention over conventional techniques. The present invention can provide specific embodiments with the capability of automatically adjusting astigmatism and a focal point at a relatively high speed and with a relatively high degree of precision by using a small number of particle images obtained as a result of radiation of a converged charged particle beam to a sample in a scanning operation without inflicting a damage on the sample. In addition, select embodiments according to the present invention can also provide the capability of carrying out an automatic inspection of a pattern defect, such as a foreign substance on an object substrate, or an automatic measurement of dimensions of a pattern on the object substance. In specific embodiments, such tasks can be carried out with a relatively high degree of precision while sustaining the quality of a particle image in a relatively stable manner over a relatively long period of time by using the particle image. The particle image is obtained as a result of radiation of a converged charged particle beam while adjusting astigmatism and focal point thereof automatically and at a relatively high speed and with a relatively high degree of precision.
Select embodiments according to the present invention can provide a charged particle beam apparatus and an automatic astigmatism adjustment method capable of automatically correcting astigmatism in a relatively short period of time for a variety of samples. Such embodiments according to the invention can find a plurality of astigmatism correction quantities at the same time from a small number of 2 dimensional images. In specific embodiments, damage inflicted on samples can be kept to a minimum.
Furthermore, some specific embodiments according to the present invention can provide a charged particle beam apparatus capable of carrying out inspections, measurements and fabrications with a relatively high degree of reliability and in a relatively stable manner over a relatively long period of time. The quality of a charge article image obtained from an object substrate as a result of automatic correction of stigmatism and a focal point of a charged particle beam optical system is improved in some embodiments.
Moreover, many specific embodiments according to the present invention can provide a sample used for an adjustment of astigmatism and a focal point of a charged particle beam and suitable for an automatic correction of the astigmatism and the focal point in a relatively short period of time by suppressing damages inflicted on the sample to a minimum in a charged particle beam optical system.
Yet further, some specific embodiments according to the present invention can provide an automatic astigmatism adjustment method capable of automatically correcting astigmatism and a focal point in a relatively short period of time from a 2 dimensional particle image and to provide a sample for the method.
These and other benefits are described throughout the present specification. A further understanding of the nature and advantages of the invention herein may be realized by reference to the remaining portions of the specification and the attached drawings.